Designs that use DC permanent magnet motors often make use of filters to eliminate noise from arcing caused by the commutation of the brushes in the motor. Generally a LC low pass filter is used to address the noise issue. In this filter configuration an inductance is placed in series between the switched power feed to the motor and the brush and a capacitor is placed from the switched power feed to ground. This filter topology is effective at filtering brush noise when the motor is running. However, due to the capacitance from the switched power feed to ground and the associated inrush current, this filter configuration can create excessive stress on relays used to control the motor. Moreover, a “pop” in the AM radio each time the motor is switched may result. In cases where the power to the motor is switched often, such as in the operation of intermittent windshield wipers, a repetitive “popping” noise in the AM radio may result. The strength of the “popping” depends on the positioning of the vehicle antenna with respect to the motor and the motor wiring harness, the radio and the strength of the station being received. Intermittent wipers can switch as often as once every two seconds so the “popping” may be unacceptable.
Since the problem is caused by current surges through the wiper wiring harness to support charging of the capacitors in the filter network as well as arcing due to the inductive nature of the motor load, a typical countermeasure has been to reduce the size of the filter capacitors or insert series resistance. The use of either of these methods will cause an increase in brush noise and Electromagnetic Interference (EMI). Typically a balance is struck between acceptable levels of brush noise and AM radio pop. If the level of noise is unacceptable more expensive semiconductor designs that allow current surges to be controlled are used.
Therefore there is a need for a new and improved system and method for suppressing brush noise. The new system and method should eliminate the capacitor surge current but retain the effectiveness of the filter for suppressing brush noise and EMI.